The present invention relates to a brushless DC motor, and in particular relates to technology for limiting variation in the capacity to detect the rotational position of the rotor.
Brushless DC motors comprise a stator having drive coils on the teeth of a core, a rotor having a magnet, and a sensor board having magnetic sensors that detect the position of the magnetic poles in this rotor. The rotor core is rotated by generating a rotating magnetic field, by way of successively electrifying each of the drive coils on the stator in accordance with the positions of the magnetic poles in the rotor, which are detected by this magnetic sensor.
With such a brushless DC motor, it is necessary to limit variation in the capacity to detect the rotational position of the rotor and, for this reason it is necessary to limit variation in the axial distance between the magnetic sensor and the magnet, while also limiting variation in the relative positions in the circumferential direction (rotational direction) of the teeth on the stator core and the magnetic sensors.
In terms of structures for positioning this magnetic sensor, in Japanese Patent Laid-Open Publication No. 2008-54390, an engagement claw that extends in the axial direction is provided at the end face of an electrically insulating member (insulator) that covers the stator, this engagement claw engaging with the end of the sensor board by way of being elastically deformed. Furthermore, anti-turn projections are provided along the inner face of the engagement claw in the axial direction, and the sensor board is rotationally positioned around a rotary shaft by advancing these anti-turn projections into positioning recesses that are provided at the peripheral edge of the sensor board.
With the magnetic sensor positioning structure described in Japanese Patent Laid-Open Publication No. 2008-54390, the axial position of the sensor board can be made constant and the variation in the axial distance between the magnetic sensor and the magnet can be suppressed. Furthermore, because the position of the sensor board is fixed in the circumferential direction by the anti-turn projections, the relative positions in the circumferential direction of the teeth, on the stator core and the magnetic sensor, can be made constant.
However, with the brushless DC motor described in Japanese Patent Laid-Open Publication No. 2008-54390, because anti-turn projections are provided on the inner face of the engagement claws of the electrically insulating member (insulator) which engage the sensor board, it is possible that, when the engagement claws are engaged with the sensor board by way of being elastically deformed, a slight position shift will occur between the anti-turn projections and the positioning recesses in the sensor board, in the circumferential direction.
If such a position shift occurs, a large force will be applied to the long thin engagement claws of the electrically insulating member, which may result in damage to the engagement claws. Furthermore, in order to correctly fit the anti-turn projections into the positioning recesses, it is necessary that these be rotated relative to each other. Thus, the positioning of the sensor board in the circumferential direction is troublesome, and there is a risk of damaging the engagement claws or the sensor board when the member is rotated.
Furthermore, in the brushless DC motor described in Japanese Patent Laid-Open Publication No. 2008-54390, because anti-turn projections are provided on the inner faces of the engagement claws that engage the sensor substrate, forming high anti-turn projections is detrimental to the elasticity of the engagement claws, and thus engagement with the sensor board is not easy. Meanwhile, if low anti-turn projections are formed, while this is not detrimental to the elasticity of the engagement claw, the anti-turn function may not be sufficient.